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Old 09-10-2013, 09:58 PM
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Excessive bouts of smoke on highway

I took my new-to-me '76 240D on a little 2 hour road trip from Phoenix to Flagstaff on Saturday. Three times on the way up while going 55mph, I would feel a slight loss of engine power, only to look in the rear view mirror to see a huge plume of dark black soot come out the back end of the car and smoke-screen the poor person behind me. Otherwise the trip there and back was uneventful. What could be the cause of that? I had an oil change recently done and it starts/drives fine. Thanks - Mike

133,000 miles

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Old 09-10-2013, 10:01 PM
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A problem with the intake butterfly valve/IP diaphragm?
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Old 09-10-2013, 11:11 PM
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I don't know, but I have a theory that I'd like to tee up for comment. The theory is that under "normal" throttle conditions over extended periods of time, soot builds up in the exhaust and muffler. Then under high load and high rpms the build up of soot gets blown out all at once. Under the right conditions it might be possible for actual clumps of soot to let loose and break up exiting the exhaust producing a really heavy black cloud.

That's my theory and I reserve the right to be completely wrong.
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Old 09-10-2013, 11:15 PM
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You probably knocked loose some built up carbon in the exhaust system, since the smoke was black. Check your oil for water, maybe open up your valve cover to check for water and check the coolant for oil.

You might check to see if the IP governor diaphragm is holding vacuum (I just went through this myself) but if it is torn or has a leak, your smoke wouldn't be black unless you sucked all the oil out of the IP into the engine. Check your IP oil while you're at it and the hose that goes from the IP to the throttle body. If it's black, you have a torn diaphragm. It's an easy fix. Finding the parts is the hard part. Mercedes has them, they are special order items.

Considering that you had power before and after your sooty outbursts, and you didn't have a runaway motor, you probably just cleared a bit of built up carbon in the muffler or resonator.

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Old 09-11-2013, 01:04 AM
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Time for new injectors....I would think, that your injectors aren't misting anymore and rather peeing diesel in....thus extra fuel....thus smoke....or someone has played with the injection pump...

Could also be a torn diaghram in the vacuum pump but this would lead to loss of oil....

Has your oil bath air filter ever been cleaned? And oil added?

When they did the oil change did they use diesel rated 15w-40 and they put both pieces of the oil filter in?

Remember diesels don't like high altitude, and you car doesn't have an ada or alda.....so a loss of power and black smoke maybe normal....

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